How to secure data communication is an important problem in wireless sensor networks
(WSNs). General solutions to the problem are to encrypt the packet payload with
symmetric keys. But those solutions only prevent the packet content from being snooped
or tampered. Adversaries still can learn of network topology by the traffic analysis attack
for starting devastating attacks such as the denial-of-service attack and the like. In this
paper, we propose an anonymous path routing (APR) protocol for WSNs. In APR, data are
encrypted by pair-wise keys and transmitted with anonyms between neighboring sensor
nodes and anonyms between the source and destination nodes of a multi-hop communication
path. The encryption prevents adversaries from disclosing the data, and the anonymous
communication prevents adversaries from observing the relation of the packets for
further attacks. We implement APR on the MICAz platform to evaluate its overheads for
demonstrating its applicability in practical WSNs.